The interferons (IFNs) are powerful anti-viral cytokines that can limit virus replication by directly triggering the death of infected cells. We have recently reported the identification of a new pathway of programmed necrosis activated by IFNs, but how this pathway is regulated during antiviral innate-immune responses remains unclear. In this proposal, we outline the discovery of a putative new checkpoint that specifically licenses the necrotic cell death of virus-infected cells by IFNs. The goal of this proposal is to identify how this checkpoint is regulated, and what its role is during acute RNA virus infections in vivo. We have found that IFNs activate RIP1/RIP3 kinase-mediated necrosis only when the adaptor protein FADD is disabled. In uninfected cells, FADD sequesters RIP1 to block necrosis, but upon an acute RNA virus infection, FADD becomes phosphorylated on a conserved serine (S191 in mice or S194 in humans) and can no longer inhibit RIP1. Under these circumstances, IFNs activate RIP kinases and induce necrosis. These findings represent the first complete outline of an IFN-activated necrosis pathway and identify phosphorylation of FADD as a putative new innate-immune cell-fate checkpoint. In the first Aim of this proposal, we will identify how FADD is phosphorylated during virus infections, and how this event licenses IFN-activated necrosis. In the second Aim, we will use a unique mouse model in which FADD cannot be phosphorylated to identify the role of IFN- triggered necrotic death during acute respiratory infection by RNA viruses (including influenza A). The successful completion of these experiments stands to define a new arm of the IFN antiviral response.